Microlithography is used for the manufacture of micro-structured components such as for example integrated circuits or liquid crystal displays (LCDs). The microlithography process is performed in a so-called projection exposure apparatus which includes an illumination system and a projection objective. The image of a mask (also called a reticle) which is illuminated via the illumination system is projected via the projection objective onto a substrate (for example a silicon wafer) which is coated with a light-sensitive coating (for example a photoresist) and set up in the image plane of the projection objective, whereby the mask structure is transferred to the light-sensitive coating of the substrate.
In order to achieve an imaging process which is rich in contrast and of high resolution, the providing of defined polarization distributions is desirable in both the illumination system and the projection objective. However, a problem encountered here is that an initially provided or adjusted polarization distribution in a bundle of light rays passing through the respective optical system is modified due to polarization-modulating effects, in particular stress-induced birefringence being induced by optical mounts, gravitation-induced birefringence, intrinsic birefringence or natural birefringence in the material of the optical components as e.g. lenses, as well as due to polarization-manipulating effects of dielectric layers. These effects result in errors in telecentry and losses in contrast in the optical imaging process due to the introduction of retardations of varying strength into the optical system. The term “retardation” in this context means the difference between the respective optical path lengths for two orthogonal (i.e. mutually perpendicular) states of polarization.
U.S. Pat. No. 6,252,712 B1 discloses an optical system with at least one optical element that causes a disturbance of the distribution of polarization over the cross section of a light beam, wherein at least one birefringent optical element is provided with a thickness that varies irregularly over the cross section such that the disturbance of the distribution of polarization is at least partially compensated. To this, the at least one birefringent optical element is provided with a free form surface via ion beam processing, and at least one plate of isotropic material with a thickness varying over the cross section is used to equalize disturbances of the wave front.
EP 1 022 617 A2 inter alia discloses an optical correction plate to compensate for the residual distortion in a microlithographic projection exposure apparatus, wherein two surfaces of the plate have an identical aspherical profile. In a method of manufacturing the correction plate a plane-parallel plate is sucked against a base surface with the desired aspherical profile, after which the exposed surface is worked flat.
WO 2005/069081 A2 inter alia discloses a polarization-modulating optical element which includes an optically active crystal with an optical axis and which has a thickness profile which, as measured in the direction of the optical axis, is variable.